The present invention generally relates to a piezoelectric filter and, more particularly, to a piezoelectric filter of a type having three terminal elements.
There is known a piezoelectric filter of a type having three terminal elements, an example of which is disclosed in Japanese Laid-open Patent Publication No. 52-110547 and Japanese Laid-open Utility Model Publications No. 52-122630, No. 52-122631 and No. 52-122632, all of which were laid open to public inspection in 1977, and shown in FIGS. 1 to 8 of the accompanying drawings. Referring to FIGS. 1 to 7, the prior art piezoelectric filter comprises a piezoelectric substrate 1 of a square shape having one face deposited or coated with an outer electrode layer 2 and a center electrode layer 3 concentric with and electrically insulated from the outer electrode layer 2, as best shown in FIG. 1, the opposite face of said piezoelectric substrate 1 being deposited or coated with a common ground electrode layer 4, as best shown in FIG. 2. The substrate assembly having the piezoelectric substrate 1 and the electrode layers 2, 3 and 4 is shown in sectional view in FIG. 3.
Shown in FIG. 4(a) is a first elongated terminal member 5 having a pair of electroconductive elastic tongues 5a and 5b extending in the same direction at right angles to the first elongated terminal member 5 and parallel to each other, said elastic tongues 5a and 5b being spaced from each other a predetermined distance which corresponds to the space between the opposite side portions of the square shaped surrounding electrode layer 2. As best shown in FIG. 4(b), each of the elastic tongues 5a and 5b is bent to provide a contact area 6a or 6b which, when in an assembled condition as shown in FIG. 7 and as will be described later, contact a respective area P1 or P2 of the opposite side portions of the surrounding electrode layer 2.
Shown in FIG. 5(a) is a second terminal member 7 having an electroconductive arm 7a protruding laterally from the terminal member 7 and having a contact protuberance 8 defined therein as best shown in FIG. 5(b). The electroconductive arm 7a is of such a size that, when in the assembled condition as shown in FIG. 7, it can be located, electrically insulated from the first terminal member 5, within a space defined by the first terminal member 5 and its elastic tongues 5a and 5b with the contact protuberance 8 held in contact with the center electrode layer 3.
Shown in FIG. 6(a) is a common terminal member 9 comprising a long and a short electroconductive bar 9a and 9b, respectively, which extend at right angles to each other to assume a substantially cross shape. Common terminal member 9 has a contact protuberance 10 defined therein and protruding outwards from the intersection area of the bars 9a and 9b, as best shown in FIG. 6(b). This common terminal member 9 is, as best shown in FIG. 6(b), substantially spherically curved with the contact protuberance 10 located at the top of the spherical shape of the common terminal member 9 so that the common terminal member 9 is resilient against pressure exerted on it in a direction opposite to the direction of projection of the contact protuberance 10.
These component parts described with reference to and shown in FIGS. 1 to 6 are assembled together in a manner as shown in FIG. 7 and housed in a casing 14 that may comprise similar casing halves 14a and 14b of a substantially container-like shape. More specifically, the substrate assembly including the piezoelectric substrate 1 and the electrode layers 2, 3 and 4, is supported in position within the casing 14 by means of the terminal members 5, 7 and 9 in such a manner that a resilient force is exerted by the terminal member 9 on the substrate assembly through the contact protuberance 10, which contacts the ground electrode layer 4, while the contact protuberance 8 of the terminal member 7 is held in tight contact with the center electrode layer 3. Thus, by the resilient action of the terminal member 9, the substrate assembly is firmly sandwiched in position between the contact protuberances 10 and 8 within the casing 14. Furthermore, the contact areas 6a and 6b of the elastic tongues 5a and 5b of the terminal member 5 are held in contact with the respective contact areas P1 and P2 of the opposite side portions of the outer electrode layer 2 by the resilient action of the elastic tongues 5a and 5b. It is to be noted that the resilient force exerted by the elastic tongues 5a and 5b is smaller than the resilient force exerted by the terminal member 9. More specifically, in the prior art filter of the construction particularly shown in FIG. 7, the resilient force exerted by the elastic tongues 5a and 5b is so selected as to be between 1/4 and 1/2 of the resilient force exerted by the terminal member 9. The reason for this is that, since the contact areas P1 and P2 correspond to loops of mechanical vibration of the piezoelectric substrate 1, when the contact areas P1 and P2 are strongly pressed by the contact areas 6a and 6b, respectively, the mode of vibration occurring in the piezoelectric substrate 1 will adversely be affected to such an extent as to cause both the insertion loss and the center frequency to vary as shown in FIG. 8. It is to be noted that in the graph of FIG. 8 the term "contact pressure ratio" means the ratio of the pressure between contact of the contact areas 6a, 6b and the contact areas P1, P2 to the pressure of contact between the contact protuberance 10 and the ground electrode layer 4 in alignment with the contact protuberance 8.
In the prior art piezoelectric filter of the construction described with reference to and shown in FIGS. 1 to 7, selection of the optimum resilience to be exerted by the elastic tongues 5a and 5b involves a certain difficulty. If their resilience is relatively but not excessively low, the piezoelectric filter will exhibit favorable performance characteristics, but its resistance to vibration and impact will be lowered. Conversely, if the resilience exerted by the terminal member 5 is high, the resistance of the filter to vibration and impact will be improved, but the filter will no longer exhibit favorable performance characteristics. In addition, the complexity of the design and manufacture of the prior art piezoelectric filter significantly increases manufacturing cost.